lint: initialize mypy
    
    Ignore the .mypy_cache/ directories.
    
    Also use global 'ignore_missing_imports = True', otherwise we'll see
    a new errors like this:
    
      Error: MYPY_ERROR:
      coprs_frontend/coprs/auth.py:8: mypy[error]: Skipping analyzing "ldap": module is installed, but missing library stubs or py.typed marker
    
      Error: MYPY_ERROR:
      coprs_frontend/coprs/auth.py:9: mypy[error]: Skipping analyzing "openid_teams.teams": module is installed, but missing library stubs or py.typed marker
    
      Error: MYPY_NOTE:
      coprs_frontend/coprs/auth.py:9: mypy[note]: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
    
    We can find a better way to handle similar problems in the future.
    
    Merges: #2353